A generally rectangular intra-spinous spacer preferably made of bone material advantageously has a low potential for bone fusion. The spacer has depressions sized and positioned on opposite sides to receive spinal processes. The depressions reduce the amount of bone-to-spinal process contact. A fixation strap of the spacer secured either to both the superior and inferior spinous processes or to only one of the superior or inferior processes laterally maintains the implanted position of the spacer and limits either flexion and extension, just flexion, or just extension.